Composite structures in aircraft typically utilize a tough skin surface supported by a lightweight core material. Development efforts to increase the strength/weight ratio of the core have resulted in cellular plastic structures such as rigid expanded foam of random cell pattern. Superior structural properties have been realized in cores formed in a geometric honeycomb pattern of hexagonal ducts, achieving very light weight due to the high percentage of air volume: 90% to 98%. Such a core, when sandwiched between two skins, forms a directional structure possessing a uniform crushing strength under compression.
In known art, such cellular or ducted cores are commonly made from thermosetting resins, which are plastics which solidify when first heated under pressure, and which cannot be remelted or remolded, as opposed to thermoplastic resins, which are materials with linear macromolecular structure that will repeatedly soften when heated and harden when cooled.
As utilization of such structures is expanded to include areas previously avoided due to structural demands and temperature, vibration and impact loading environments, new composite matrices are required. Thermosetting resins, commonly used, in most cases, lack the toughness and stability needed for these applications.
New thermoplastic materials offer improved properties; for example composite skin-surfaced structures having honeycomb cores made from thermoplastic preimpregnated fiber material provide excellent impact strength and damage tolerance. However, by their nature, the new thermoplastic materials require new and unconventional processing methods. As opposed to conventional thermosetting processes where sticky and viscous fluids are saturated into reinforcing fiber forms to be cured by catalysts and heat, thermoplastics which have no cure cycle are hard and "boardy" initially, and have to be melted at high temperatures to be worked to the desired shapes. Thus completely different processing schemes are required for thermoplastics than those that have been developed for thermosets. This also holds true for the honeycomb core which is used to give light composite aircraft parts large moments of inertia to multiply stiffness and strength without proportional increases in weight.
In known art, thermoset honeycomb material is made by a process that takes advantage of the flexibility of the reinforcing fabric before it is impregnated with resin. It is bonded and then expanded into hexagon honeycomb structure while it is soft, then wash coated with resin which is subsequently cured to give it its stiffness.
In contrast, thermoplastics, utilized in the present invention for their superior ultimate properties, have no soft stage, and they are too viscous to be wash coated or by some other means saturated into the fabric after bonding the sheets together. The practical options for bonding thermoplastic fiber material are limited by the high degree of stiffness of thermoplastic resin impregnated fiber reinforcement. Therefore, pre-forming of the thermoplastic fiber reinforced material into a ducted honeycomb structural pattern has been selected as the method for producing strong lightweight core material in the present invention, which addresses new processing methods for realizing the full benefits of the superior ultimate properties of such structure.
Thermoplastic preimpregnated fiber material is available both in ribbon (continuous woven fabric sheet) and yarn form.
The first step in processing thermoplastic preimpregnated fiber material into ducted core structures is to preform the thermoplastic material, typically in sheets which can be stacked and thermal fusion bonded in the desired duct pattern.
Thermal fusion bonding processes for this type of core may be divided into two main classifications:
(a) block processing where an entire honeycomb block is bonded in a single bonding cycle, and PA1 (b) layer processing where each sheet is bonded to the previously added sheet in a separate bonding cycle for each sheet as a stack is thus built up progressively. PA1 (1) the cycle time is long due to the labor required to prepare, stack and remove the mandrels: for 3/16" (4.76 mm) hex ducts, 1100 mandrels are required for a cube core of 6" (15.24 cm) per side; PA1 (2) the preformed corrugated sheets have an inherent curvature and springiness which make the stacking process difficult and slow; PA1 (3) removing the mandrels from the bonded honeycomb block presents major difficulties, requiring a special long stroke pneumatic press with a supported long thin pin to push them out individually, a slow, skill-intensive and risky process since the mandrels tend to become partially bonded in place despite the use of release agents, and any mushrooming of the mandrel ends by the press pin is likely to damage the honeycomb duct structure; PA1 (4) heat penetration throughout the block is slow and must be carefully monitored with multiple embedded thermocouples to ensure complete bonding in all regions of the block; and PA1 (5) practically all of the aforementioned difficulties become sharply aggravated when it is attempted to increase the size of the workpiece, thus there are unacceptable limits on producible block size. PA1 (a) supporting at least one assembly of extended narrow, elongate, uniformly spaced, mandrels or induction heating rods for simultaneous retractable movement along a horizontal plane, the rods being dimensioned and spaced to extend between adjacent corrugations on the lower surface of the bottom corrugated sheet being bonded and substantially throughout the length of the corrugations; PA1 (b) inserting a first corrugated sheet in horizontal position over the extended heating rods so that adjacent heating rods nest between adjacent corrugations on the lower surface to support the first sheet substantially across its width; PA1 (c) inserting a second corrugated sheet in aligned horizontal position over said first sheet so that the peaks or planar facets of the corrugations on the lower surface of the second sheet are aligned for overall surface contact with the peaks or planar facets of the corrugations on the upper surface of the first sheet; PA1 (d) lowering a heating/cooling platen against the upper surface of the second sheet, the platen comprising a plurality of narrow, elongate, uniformly spaced heating/cooling contact surfaces or ridges which are dimensioned and spaced to extend between adjacent corrugations on the upper surface of the second sheet, substantially throughout the length thereof, to press the contacting co-planar facets or peaks of the corrugations of the second and first sheets between a said ridge and an induction heating rod; PA1 (e) inducing an impulse current through the induction heating rods, contacting facets or peaks and platen ridges sufficient to heat-fuse the first and second thermoplastic sheets to each other in the areas of the contacting facets or peaks; PA1 (f) cooling the platen ridges to reduce the temperature of the heat-fused peak areas, and PA1 (g) retracting the induction heating rods to provide a honeycomb structure of the corrugated sheets, the peak or facet areas of which are fused to each other to form therebetween adjacent elongate honeycomb ducts or passages comprising the spaces between the peak surfaces on the lower surface of the first sheet and on the upper surface of the second sheet.
In a basic block process, thermoplastic preimpregnated material is preformed into duct (or half-duct) patterned sheets which are stacked into a press, along with interspersed rows of duct forming mandrels in a honeycomb pattern. With a full stack under compression, heat is applied to fusion bond all the sheets in the stack together at their interfacial facets. Then after cooling to a setting temperature, pressure is released and the mandrels are extracted, leaving a completed honeycomb block of ducted core material.
This basic block bonding process, while viable for smaller sized workpieces and developmental purposes, poses certain difficulties and limitations, which have been addressed by the present invention:
As an alternative to the block bonding process, a layer bonding process which bonds only one layer at a time offers a number of advantages such as greatly reducing the number of mandrels required, facilitating uniform bonding, and enabling production of much larger sized blocks. However certain new and different difficulties must be overcome in layer bonding.
The present invention addresses solutions to these and other problems and difficulties in seeking improved processes and apparatus for the continuous processing of thermoplastic preimpregnated fiber material into larger sized structures of ducted core material, as reflected in the following objects and summary of the invention.